The need to monitor temperature occurs in a wide variety of automated applications, such as manufacturing and transportation of goods. For example, during various phases of a fabrication process, it may be important for the workpiece to remain within a limited temperature range, while food or medicine may be spoiled during shipment if exposed to temperatures above a particular maximum. While electronic temperature measurement is fast and accurate, traditional systems utilize temperature probes that are physically connected, via wires, to sensing circuitry. It is frequently not feasible or inconvenient to run wires to the desired measuring point.
In recognition of such environmental limitations, wireless temperature-monitoring systems have been developed. These typically involve associating both the temperature probe and measurement circuitry with the monitored items. Also associated with the measurement unit is a radio circuit that broadcasts the sensed temperature for remote reading. Obviously this arrangement involves considerable expense and bulk.
An alternative to these approaches is described in U.S. Pat. No. 6,025,725, which discloses an LC resonator package incorporating an electrically active material that responds to an external condition such as temperature. This response alters the resonant frequency and/or harmonic spectra of the package in a predictable fashion, thereby facilitating quantification of the external condition. While simple and inexpensive, the disclosed approach may encounter difficulties in electrically noisy or conductive environments. The LC resonators tend to operate at relatively high frequencies (&gt;1 MHz), and are therefore easily shielded.